1) The difference is motive. When Nolan redoes Batman, he does it for a compelling artistic reason. When Branagh adapts Hamlet, he does it because he feels he has a new, unique artistic vision that differs from previous adaptations, with Olivier and Gibson, of that same ever-malleable source text. When people offer a new adaptation of a literary text, it is almost invariably because the artist believes they have a new and interesting way of perceiving that world. When someone brings back a character that has previously only existed in another movie, the motive is economic. It's a producer's motive. It's about safety and brand recognition. If indeed Khan gets reused, it won't be because Abrams' muse was speaking to him about some world-shattering new artistic vision - it will be because he knows it will get butts in the seats. It's a base motive, and it's cowardly. .
I'm impressed by your ability to read people's minds and know whether their motives are "artistic" or "base."
I can only assume based on experience. And experience seems to indicate that when movies are redone (rather than literary sources being readapted), the new version very, very rarely has an original enough or creative enough new perspective to warrant redoing the thing. As a result, I can only assume that the motive was economical. (See any of the hundreds of remakes of horror movies from the last decade, for pertinent examples.) Off-hand, the only example I can think of where a remake of a movie, rather than a readaptation of a literary text, had something different and original enough to say to warrant the remake, is the Denzel Washington version of Manchurian Candidate. But, as I say, 90% of the time, the resulting piece clearly indicates that the motive was economical.
In any case, I have read the many responses to my original point, and many of the counter-arguments sound valid. Is a remake of a visual, commercial medium like film somehow a lesser art form than a new adaptation of a literary text? My gut tells me so. My gut also tells me that novelizations of movies are a lesser art form than filmed versions of novels, and granted, perhaps I don't have the logical arguments to explain why precisely my gut feels that way. Is it arbitrary, this distinction? I don't know. I don't think so. And I'm sure, nevertheless, that most of us will agree that the novelization of, say, Back to the Future II, is not an attempt to create great art.
My argument, even if based on my gut feeling, is not based on nostalgia. I have no nostalgia for the original Star Trek, having not been born yet when it first aired. It is based on my understanding of how different mediums work. Characters in literary texts, it has been said, are nothing more than "bags of bones," waiting for some actor, director, costume person, etc, to bring them to life. Characters that originate on screen, however, don't have that kaleidoscope quality. Somehow, casting a new Miss Havisham feels like a valid artistic endeavor, while recasting Khan feels like someone playing dress up. If there's anyone here who senses what I'm saying, and can perhaps find the logic to support the gut feeling I'm describing, then I'll be happy to hear it.
On another front, anyone arguing that Khan is a highly literary tragic character is clearly stretching things a bit. Wrath of Khan is by no means a great movie because of the intensity of the villain (that misreading of the film has lead to two or three disastrous Star Trek movies in a row now, trying to recapture a movie's success without having a clue why it works so well in the first place.) Wrath of Khan is fantastic because of the relationships between the three leads, because of the intense connection between theme and action, because of the depth of those themes about growing old and regret and feeling out of touch, and because of the tone of the film. Khan may be iconic, sure, but iconic status by no means implies high literary merit. Jim Carrey singing out of his ass is also iconic, and it's admittedly funny as hell, but there is no high artistic ambition there.
Abrams needs to reinvent what Star Trek MEANS. He needs to reinvent the kinds of stories the franchise can tell. He needs to tell stories that the franchise has never told before. Otherwise, the whole project is nothing but a massive plunge into nostalgia.